1. Field of the Invention
This invention relates to the introduction of fluids into subterranean formations, more particularly to acid treatment of the formations.
2. Description of the Art
Acid treatment, or acidizing, is a very wellknown method for increasing or restoring the permeability of porous subterranean formations, for the purpose of facilitating the flow of fluids, such as crude oil, natural gas, geothermal fluids, and the like, through the formation. In a typical treatment, an acid or mixture of acids is introduced into a formation, through a well which penetrates the formation, using sufficient pressure to obtain a desired distance of penetration into the formation. During the treatment, passageways for the flow of fluids are enlarged and a certain amount of new passageways may be formed. Acidizing effects depend primarily upon the chemical nature of the formation and the acid used, and typically are shown by an increase in fluid production (or fluid injection) rate through the well, after acidizing has been conducted.
Materials which are attacked by the introduced acid can be original components of the formation, and also can be subsequently deposited by well drilling, production, and injection operations after the well has been completed. Very common carbonate-containing formations and clogging deposits are frequently treated with hydrochloric acid. Hydrofluoric acid, a mixture of hydrochloric and hydrofluoric acids, or a mixture of hydrochloric acid and fluoride salts, all of which can be generally described by the term "mud acid," is commonly utilized for removing siliceous materials.
Strong, aqueous mineral acids, however, react almost instantaneously with the first reactive materials encountered as the acids are introduced into a formation. Frequently, these reactive materials are finely divided particles from the formation itself. One undesired result is the consumption of large amounts of acid very near the wellbore, through which acid is injected, and a limited acid penetration into the formation. Adequate penetration depths are achieved only by the use of very large quantities of acid, which itself causes added corrosion problems for well tubing and other subsurface equipment, due to their increased exposure to acidic materials.
These disadvantages have been addressed, at least in part, by adding various materials to the acid which make the acid less reactive initially, but which maintain reactivity over a prolonged period as the acid moves into a formation. Such added materials generally form: (1) emulsions with aqueous acid solutions; or (2) polymeric thickened or gelled acid compositions. Factors such as formation heat are used to decompose the emulsion or polymer, releasing reactive acid in a more or less gradual manner.
U.S. Pat. No. 4,479,543 to L. J. Kalfayan and D. R. Watkins describes a deeper penetrating acidizing method, in which the acid solution injection is preceded by injecting a slug of an organosilane or an ester of an organosilane. Preferably, the organosilane or ester is injected as a solution in a hydrocarbon carrier liquid, to prevent water contact before the material enters the formation to be treated, since water-reacted silane material was thought to penetrate a formation only to a limited extent. One possible mechanism proposed to explain the enhanced acidizing effects observed from use of the method is the coating of formation fines with silane material, which polymerizes and protects the fines against acid attack.
The previously discussed methods for acidizing procedure improvement have the common disadvantage of increased complexity, as compared to normal, simple acid injection techniques. Additional time, energy, equipment, and materials are required to form emulsions which are stable enough for acidizing, and polymer-thickened acids are more difficult to inject into a formation. Multiple injection procedures also suffer somewhat from increased labor and equipment utilization costs.